Various processes are known in the art that involve the production of metal oxides from vaporous reactants. Such processes require a feedstock solution, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site, and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected on any deposition receptor in any number of ways ranging from a collection chamber to a rotating mandrel. It may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles and burners.
Much of the initial research that led to the development of such processes focused on the production of bulk silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of 200-300 millimeters of mercury (mm Hg) at temperatures below 100.degree. C. would be useful for making such bulk silica. The high vapor pressure of silicon tetrachloride (SiCl.sub.4) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other, is responsible for the presently accepted use of SiCl.sub.4, GeCl.sub.4, POCl.sub.3, and BCl.sub.3 as vapor sources, even though these materials have certain chemically undesirable properties.
Silicon, germanium, zirconium, and titanium are metals often used in halide form as vaporous reactants in forming metal oxide glasses. However, SiCl.sub.4 has been the industry standard over the years for the production of high purity silica glasses. As disclosed in U.S. Pat. No. 3,698,936, one of several reactions may be employed to produce high purity silica via oxidation of SiCl.sub.4 ; namely: EQU SiCl.sub.4 +O.sub.2.fwdarw.SiO.sub.2 +2Cl.sub.2, (1) EQU SiCl.sub.4 +2/3O.sub.3.fwdarw.SiO.sub.2 +2Cl.sub.2, or, (2) EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl, (3)
whereby burners or jet assemblies are utilized in feeding the reactant gases and vapors to a reaction space. It should be noted that reaction (2) rarely occurs or is used. There are inherent economic disadvantages to each of these reactions. Moreover, these reactions, which oxidize SiCl.sub.4 through pyrolysis and hydrolysis, have the disadvantage of producing chlorine or a very strong acid by-product.
While the first two reactions occur theoretically, an auxiliary fuel is generally needed to achieve pyrolytic temperature. The hydrolysis of SiCl.sub.4 results in the formation of hydrochloric acid (HCl), a by-product that is detrimental not only to many deposition substrates and to reaction equipment but also is harmful to the environment. Emission abatement systems have proven to be very expensive due to down-time, loss, and maintenance of equipment caused by the corrosiveness of HCl.
Notwithstanding the problems with handling and disposal of the HCl by-product, the third reaction, hydrolysis of SiCl.sub.4, tends to be the preferred commercial method of producing silica for economic reasons.
Though hydrolysis of SiCl.sub.4 has been the preference of industry for producing high purity silica over the years, the enhanced global sensitivity to environmental protection has led to more strict government regulation of point source emissions, prompting a search for less environmentally pernicious feedstocks. Point source emission regulations require that HCl, the by-product of hydrolyzing SiCl.sub.4, as well as many particulate pollutants be cleansed from exhaust gases prior to their release into the atmosphere. The economic consequences of meeting these regulations have made commercial production of silica from halide-based feedstocks less attractive to industry.
As an alternative, high purity quartz or silica has also been produced by thermal decomposition and oxidation of silanes. However, this requires taking safety measures in handling because of the violent reaction that results from the introduction of air into a closed container of silanes. Silanes react with carbon dioxide, nitrous oxide, oxygen, or water to produce high purity materials that are potentially useful in producing, among other things, semiconductor devices. However, silanes have proven to be much too expensive and reactive to be considered for commercial use except possibly for small scale applications requiring extremely high purity.
A number of-patents describe the production of high purity metal oxides, particularly silica, from a chloride-based feedstock. These patents disclose equipment with a number of burner arrangements and feedstock delivery systems to achieve oxidation of a metal chloride through flame hydrolysis or pyrolysis. Illustrative of this is U.S. Pat. No. 4,491,604 to Lesk et al., where trichlorosilane, dichlorosilane, and silicon tetrachloride are flame hydrolyzed to form soot, and U.S. Pat. No. 3,666,414 to Bayer, where silicon halides such as trichlorosilane or chloroform are flame hydrolyzed. In similar processes, U.S. Pat. Nos. 3,486,913 to Zirngibl ("Zirngibl") and 2,269,059 to McLachlan ("McLachlan") teach oxidation of halides. Volatilized inorganic halide components such as TiCl.sub.4, CrCl.sub.3, CrO.sub.2 Cl.sub.2, SiCl.sub.4, AlCl.sub.3, ZrCl.sub.4, FeCl.sub.2, FeCl.sub.3, ZnCl.sub.2, or SnCl.sub.4 that are oxidized with air, steam, or oxygen are employed in Zirngibl, while silicon halides, ethyl silicate, methyl borate, TiCl.sub.4, AlCl.sub.3, and ZrCl.sub.4 are used by McLachlan.
U.S. Pat. No. 3,416,890 to Best et al. discloses a process for preparing finely-divided metal or metalloid oxides by the decomposition of a metal or metalloid perhalide in a flame produced by the combustion of an oxidizing gas and an auxiliary fuel, such as carbon disulfide, carbon selenide sulfide, thiophosgene, or other hydrogen-free compounds containing sulfur bonded directly to carbon.
U.S. Pat. No. 2,239,551 to Dalton discloses a method of making glass by decomposing a gaseous mixture of glass-forming compounds in a flame of combustible gas. The mixture is used in the formation of anhydrous oxides of silicon, aluminum, and boron. Decomposable compounds such as ethyl or methyl silicate, trichlorosilane, and silicon tetrafluoride may be substituted for silicon tetrachloride; methyl borate or boron hydride may be substituted for boron fluoride, etc.
U.S. Pat. No. 2,326,059 to Nordberg details a technique for making silica-rich ultra-low expansion glass by vaporizing tetrachlorides of Si and Ti into the gas stream of an oxy-gas burner, depositing the resultant mixture to make a preform, vitrifying the preform at 1500.degree. C. to make an opal glass, and firing the opal preform at a higher temperature to cause it to become transparent.
U.S. Pat. No. 2,272,342 to Hyde discloses a method of producing glass articles containing vitreous silica by vaporizing a hydrolyzable compound of silicon such as silicon chloride, trichlorosilane, methyl silicate, ethyl silicate, silicon fluoride, or mixtures thereof, using a water bath. The silicon compound vapor is hydrolyzed by water vapor in the flame of a burner, and the resulting amorphous oxide is collected and subsequently sintered until a transparent glass results.
U.S. Pat. No. 4,501,602 to Miller et al. describes the production of particulate metal oxide soot through the vapor phase deposition of .beta.-diketonate complexes of metals from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, and the rare earth series of the Periodic Table.
Also cited in the art are several patents where silane compounds have been used in producing high purity silica.
Japanese Patent Application No. 90838-1985 to Okamoto et al., discloses a method of doping quartz glass by utilizing an ester silane expressed by the general formula R.sup.1.sub.n,Si (OR.sup.2).sub.4-n, and one or more dopants defined by the formulae Ge(OR.sup.3) .sub.3, B(OR.sup.3).sub.3, and PH.sub.3, where R.sup.1 is a hydrogen atom, methyl or ethyl group; R.sup.2 is a methyl or ethyl group; R.sup.3 is an univalent hydrocarbon group; and n is an integer ranging between 0 and 4. A great many organometallic compounds are disclosed, including methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, tetramethoxysilane, methyltriethoxysilane, and tetraethoxysilane.
U.S. Pat. No. 3,117,838 to Sterling describes a method of producing very pure fused quartz or silica by the combined thermal decomposition and oxidation of silanes where either carbon dioxide, nitrous oxide, or water vapor and a silane are fed into a burner or torch jet, and the flame is allowed to impinge on a carbon substrate upon which silica is deposited.
U.S. Pat. No. 4,810,673 to Freeman discloses a method of synthesizing high quality silicon oxides by chemical vapor deposition of a source gas mixture which includes a halogenated silane component and an oxygen source, namely, dichlorosilane and nitrous oxide.
U.S. Pat. No. 4,242,487 to Hasegawa et al. discloses a method of producing a heat resistant, semi-inorganic compound that is useful as a material for various heat-resistant materials by reacting an organoborosiloxane compound with at least one of the group of aliphatic polyhydric alcohols, aromatic alcohols, phenols, and aromatic carboxylic acids at 250.degree. C. to 450.degree. C. in an inert atmosphere.
As is clear from the preceding discussion, it is highly desirable for both economic and environmental reasons to find halide-free silicon compounds to replace the silicon halide feedstocks typically used to produce high purity silica glass. Such halide-free starting materials would produce carbon dioxide and water, rather than noxious and corrosive HCl, as by-products of the glass-making process.
U.S. Pat. No. 5,043,002 to Dobbins et al., the disclosure of which is hereby incorporated by reference, discloses the usefulness of polymethylsiloxanes, in particular, polymethylcyclosiloxanes such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane ("OMCTS"), and decamethylcyclopentasiloxane in a method of making fused silica glass. The method can be applied to the production of a non-porous body of silica glass doped with various oxide dopants and for the formation of optical waveguide fibers. U.S. Pat. No. 5,043,002 to Dobbins et al. also discloses the use of hexamethyldisiloxane; see also reference to hexamethyldisiloxane in Japanese Patent Application No. 1-138145.
U.S. Pat. No. 5,152,819 to Blackwell et al., the disclosure of which is hereby incorporated by reference, describes additional halide-free silicon compounds, in particular, organosilicon-nitrogen compounds having a basic Si--N--Si structure, siloxasilazones having a basic Si--N--Si--O--Si structure, and mixtures thereof, which may be used to produce high purity fused silica glass without the concomitant generation of corrosive, polluting by-products.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Patent Nos. 5,043,002 and 5,152,819, avoids the formation of HCl, some problems remain, particularly when the glass is intended for the formation of optical waveguides and high purity silica soot. Applicants have found that, in the course of delivering a vaporized polyalkylsiloxane feedstock to the burner, high molecular weight species can be deposited as a gel in the line carrying the vaporous reactants to the burner or within the burner itself. This leads to a reduction in the deposition rate of the soot preform that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that will produce defective or unusable optical waveguide fiber from the affected portions of the blank.
Forming amorphous SiO.sub.2 soot particles from a feedstock comprising a volatile silicon-containing compound typically entails vaporization of that compound prior to its introduction into a combustion burner. In the previously mentioned U.S. Pat. No. 5,043,002 to Dobbins et al., for example, a carrier gas such as nitrogen is bubbled through a silicon-containing reactant compound, preferably a halide-free compound such as octamethylcyclotetrasiloxane. A mixture of the reactant compound vapor and nitrogen is transported to the burner at the reaction site, where the reactant is combined with a gaseous fuel/oxygen mixture and combusted.
Although use of halide-free silicon compounds as feedstocks for silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, avoids the formation of HCl, as mentioned above, some problems remain, particularly when the glass is intended for the formation of high quality optical products such as optical waveguides. Applicants have found, as disclosed in copending U.S. patent application Ser. No. 08/574,961 entitled "Method for Purifying Polyalkylsiloxanes and the Resulting Products," that the presence of high boiling point impurities in, for example, a polyalkylsiloxane feedstock, can result in the formation of gel deposits in the vaporization and delivery systems carrying the vaporous reactants to the burner or within the burner itself. Such polymerizing and gelling of the siloxane feedstock inhibits the controllability and consistency of the silica manufacturing process. This problem is more prevalent when an oxidizing carrier gas such as oxygen is included in the reactant vapor stream, because oxidizers appear to catalyze polymerization of the siloxane feedstock. Such polymerization and gelling reduces the deposition rate of the soot preform that is subsequently consolidated to a blank from which an optical waveguide is formed. An additional problem encountered with forming silica soot using siloxane feedstocks is particulates of the high molecular weight, high boiling impurities may be deposited on the optical waveguide fiber blank, resulting in "defect" or "clustered defect" imperfections that adversely affect the optical and structural quality of subsequently optical waveguides formed using the silica soot.
Defects are small (i.e. 0.1 to 4.0 mm in diameter) bubbles in a glass body. They can be formed in fused silica by an impurity, such as uncombusted gelled polyalkylsiloxane. A very small particle of siloxane gel can be the initiation site for a defect. The siloxane decomposes at high temperature after being deposited on the glass body, giving off gases which cause the formation of the defect.
Thermophoresis is the process by which soot is attracted to the preform. In fact, it produces the driving force which moves the particles towards the cooler preform. The hot gases from the burner pass around the preform during laydown; the soot particles do not have sufficient momentum by combustion alone to strike the preform. Thermophoresis moves particles in a temperature gradient from hot regions to cooler regions. The burnt gases from a burner are hotter than the preform. As these gases pass around the preform, a temperature gradient is produced. Hot gas molecules have higher velocity than cold gas molecules. When hot gas molecules strike a particle, they transmit more momentum to the particle than a cold gas molecule does. Thus, particles are driven towards the colder gas molecules and, in turn, toward the preform.
Clustered defects are larger glass defects found in optical waveguide fiber preforms. They are made up of a series of defects in the form of a line or a funnel- or flower-shaped cluster. A large particle of gel can be the initiation site for a clustered defect. After the gel particle has struck the porous preform, it causes a raised area to stand out from the preform surface. Because the clustered defect is a raised site, more heat transfer passes to this site. Because of this increased heat transfer, more thermophoresis occurs at this site, causing the imperfection to grow and leave behind a string of defects. As a result of the clustered defect, the affected portion of the optical waveguide preform cannot be consolidated normally, and the consequent irregularity in the blank yields defective optical waveguide. For example, in the case of a typical 100 kilometer consolidated waveguide fiber blank, which has a diameter of 70 millimeters (mm) and a length of 0.8 meter (m), the presence of one clustered defect on the surface of the blank will typically result in the loss of 5 kilometers of optical waveguide fiber on drawing. In the case of a larger consolidated blank, the negative impact of a single clustered defect is proportionately higher. In a 250 kilometer consolidated blank, which has a diameter of 90 mm and a length of 1.8 m, one clustered defect on the surface of the blank will typically result in the loss of 8 kilometers of optical waveguide fiber on drawing.
In copending application Ser. No. 08/767,653, it was disclosed that the clustered defects could be reduced by delivering a liquid siloxane feedstock to a conversion site, atomizing the feedstock at the conversion site, and converting the atomized feedstock at the conversion site into silica. One way to atomize the feedstock at the conversion site involves pneumatically or airblast atomizing the liquid siloxane feedstock at the conversion site with a delivery gas such as an inert gas. By "pneumatic" or "airblast" atomizing, we do not mean that air must be used as the atomizing gas, and the gas can also be an inert gas such as argon, nitrogen, or helium, a combustible gas such as methane, oxygen, or a mixture of these gases.
Even though atomizing the liquid feedstock reduces clustered defects, such a liquid delivery system presents several challenges. For example, increasing the delivery gas velocity desirably produces smaller liquid droplets, which are more readily vaporized and burned in the burner flame. Smaller droplets are desirable because larger droplets cause wart-like defects ("warts") on the surface of the soot blank. In addition, smaller droplets can be more easily focused with the surrounding gases to produce a more focused deposition stream. On the other hand, increasing atomizing gas velocity adds turbulence to the burner flame, which can reduce soot capture rate and appears to be one cause of a physical soot defect known as "lizard skin." Lizard skin is a term for a rough soot blank surface.
Accordingly, it would be desirable to provide a method in which a liquid delivery system could produce a focused deposition stream containing small droplets without a high gas velocity, and in which there is low burner flame turbulence.